The Optic Nerve in Moving Pictures

Prevailing concepts of the optic nerve (ON) head (ONH) and peripapillary region have been informed by static histologic and photographic images. Consequently, most investigators have been biased to envision the ONH region as largely rigid. Sibony’s accompanying paper highlights a dynamic view, employing geometric morphometric analysis of optical coherence tomography of the ONH region in idiopathic intracranial hypertension and anterior ischemic optic neuropathy (AION). Sibony found that normal subjects exhibit small seesaw deformation of the peripapillary basement membrane layer during horizontal eye rotation, mainly displacing the temporal peripapillary region posteriorly in adduction. This effect was markedly exaggerated in papilledema due to elevated intracranial pressure (ICP), with large posterior displacement of the temporal and anterior displacement of the nasal peripapillary region in adduction; the entire pattern is reversed in abduction. This deformation is nonspecific to ONH swelling, since no exaggerated effect occurred with similar swelling due to AION. Sibony’s demonstration of ablation of the gaze-related papilledema effect by ICP reduction causally implicates elevated ICP in the exaggerated phenomenon, probably due to hydraulic stiffening of the ON sheath. These findings support accumulating evidence for significant gaze direction–related influences on ONH and peripapillary morphology. Globe tethering by the temporal ON sheath has been demonstrated by magnetic resonance imaging in normal subjects and is exaggerated in esotropia and axial myopia, where there is even occasional globe retraction. Moreover, finite element analysis has suggested that shear of the ON against the retro-orbital tissues during ocular rotation might strain the ONH. Evidence is accumulating that eye movements may produce repetitive strain injury to the ONH, and thus contribute to optic neuropathy in papilledema and normal-tension glaucoma, and perhaps even peripapillary staphyloma formation in axial myopia. This is an intriguing concept for elucidation of additional causes of optic neuropathy besides elevated intraocular pressure and ischemia.